The present invention relates to a system for a four-wheel drive vehicle in which the power transmission system is automatically changed from two-wheel drive to four-wheel drive or vice versa in accordance with driving conditions. Such a four-wheel drive vehicle is provided with a clutch for transmitting the power of an engine to two auxiliary driving wheels for establishing the four-wheel drive.
There has been proposed the four-wheel drive vehicle, a power transmission system of which is automatically changed from two-wheel driving to four-wheel driving system in accordance with driving conditions.
The four-wheel drive vehicle is provided with a slip rate detecting device which detects difference between the speed of the front wheels and rear wheels. If either the front wheels or the rear wheels slip on slippery or icy or snowy roads, the slip rate will increase, which is detected by the slip rate detecting device. The clutch is engaged in dependency on a signal from the slip detecting device, so that the vehicle is driven by all four wheels. Thus, slipping of the wheels decreases and safety of driving may be insured.
However, it is desirable for such a four-wheel drive vehicle to be able also to be driven by all four wheels regardless of the driving conditions. To meet such a requirement, a manual switch for changing from an automatically changeable four-wheel-driving transmission system to a constant four-wheel-driving transmission system is necessary.
On the other hand, during periodical inspections with respect to structure of the vehicle, engine performance, the emission control system, and other components, the main driving wheels are mounted on a free roller and the auxiliary driving wheels are fixed. However, the four-wheel drive vehicle provided with the above described automatic four-wheel drive power transmission system cannot be inspected on the free wheel when fixing the auxiliary driving wheels, because the auxiliary driving wheels are driven by the operation of the slip rate detecting device.